No. You build one version; this is the premise behind the VM environment that runs your code, which is "write once, run everywhere". If you depend on system APIs only provided by Google or Amazon, you have an additional version for each system that exclusively provides those APIs.
> consumers face a bewildering array of user interfaces.
Not in the apps themselves. Conflating the system UI with the app UI?
> They have made no real effort to fix the root problems – especially around fragmentation, security and privacy.
I'll push back on this without expending too much effort: Treble, Mainline, and the new Privacy Sandbox are actual efforts that have improved things on the security/privacy front. AndroidX makes fragmentation a non-issue beyond the most esoteric of app requirements.
> Google should make Android truly open source.
I mean, I am able to submit patches to AOSP and AndroidX, have them reviewed, and merged. There are uncountable AOSP forks out there. I'm just not sure how much more open-source you can get? Unless you mean "open governance", which I could get behind.